The nonapeptide bradykinin is released by glandular or plasma kallikreins by their action on substrates in plasma called kininogens. Kininogens of high molecular weight (HMW-) and low molecular weight (LMW-) have been recognized in the plasma of man and other species. HMW-kininogen has recently been shown to participate in the intrinsic coagulation pathway, the contact activated fibrinolytic pathway as well as the kinin-forming system (J. Clin. Invest. 56:1663, 1975). In order to study the molecular basis for the action of HMW-kininogen in coagulation, fibrinolysis and related phenomena, we propose to prepare human HMW-kininogen in highly purified form. When this is achieved, we will characterize the molecule by its physical and chemical properties. We will then study the cleavage of human HMW-kininogen by human plasma kallikrein. The fragmentation products of human HMW-kininogen will then be assayed for their ability to reconstitute impaired coagulation or fibrinolysis of human HMW-kininogen deficient plasma by standard assays. It is not yet clear whether HMW-kininogen functions as a co-factor, like coagulation factors V or VIII, or whether it has enzymatic activity. In recent studies of fragmentation products of bovine kininogens, we have shown that a glycopeptide fragment of bovine HMW-kininogen is a permeability increasing agent which acts synergistically with bradykinin (Clin. Res. 24:314A, 1976). We will ascertain whether such exists in human HMW-kininogen and study its function as well. If the human glycopeptide analog can be isolated and analyzed, the mechanism of its potentiation of bradykinin will then be studied.